A New Method for Quantification of Regurgitant Flow Rate Using Color Doppler Flow Imaging of the Flow Convergence RegionProximal to a Discrete Orifice - An In Vitro Study

While color Doppler flow mapping has yielded a quick and relatively sensitive method for visualizing the turbulent jets generated in valvular insufficiency, quantification of the degree of valvular insufficiency has been limited by the dependence of visualization of turbulent jets on hemodynamic as well as instrument-related factors. Color Doppler flow imaging, however, does have the capability of reliably showing the spatial relations of laminar flows. An area where flow accelerates proximal to a regurgitant orifice is commonly visualized on the left ventricular side of a mitral regurgitant orifice, especially when imaging is performed with high gain and a low pulse repetition frequency. This area of flow convergence, where the flow stream narrows symmetrically, can be quantified because velocity and the flow cross-sectional area change in inverse proportion along streamlines centered at the orifice. In this study, a gravity-driven constant-flow system with five sharp-edged diaphragm orifices (ranging from 2.9 to 12 mm in diameter) was imaged both parallel and perpendicular to the direction of flow through the orifice. Color Doppler flow images were produced by zero shifting so that the abrupt change in display color occurred at different velocities. This "aliasing bounary" with a known velocity and a measurable radial distance from the center of the orifice was used to determine an isovelocity hemisphere such that flow rate through the orifice was calculated as 2Wr,Xr2XVr, where r is the radial distance from the center of the orifice to the color change and V, is the velocity at which the color change was noted. Using V, values from 54 to 14 cm/sec obtained with a 3.75-MHz transducer and from 75 to 18 cm/sec obtained with a 2.5-MHz transducer, we calculated flow rates and found them to correlate with measured flow rates (r=0.94-0.99). The slope of the regression line was closest to unity when the lowest Vr and the correspondingly largest r were used in the calculation. The flow rates estimated from color Doppler flow imaging could also be used in conjunction with continuous-wave Doppler measurements of the maximal velocily of flow through the orifice to calculate orifice areas (r=0.75-0.96 correlation with measured areas). In a clinical series of 20 patients studied prospectively, radius of the flow convergence area separated patients with angiographically mild from those with moderate (p'O.OOl) and patients with angiographically moderate from those with severe (pc0.005) mitral regurgitation and showed good correlation with the angiographic severity of regurgitation (r=0.87). Color Doppler visualization of the flow convergence region is a method that appears promising for providing a calculated value of flow rate for a regurgitant orifice in the cardiovascular system. When used in conjunction with continuous-wave Doppler, color Doppler flow imaging can be used to predict the orifice area. (Circulation 1991;83:594-604)